This invention is directed generally to chemical injection spraying and more particularly to a novel and improved injection mixing apparatus for use with a spraying system.
For agricultural and/or highway chemical spraying, relatively large mobile spraying apparatus are utilized. Typically, such mobile spraying apparatus include a supply tank for the fluid to be sprayed, a pump, a flow control valve and a distribution boom fitted with a number of spray nozzles. Generally speaking, the desired spray mixture is premixed in a supply tank; i.e., at the desired, and constant, chemical concentration for a given application. Thereafter, a flow control system based on the ground speed of the vehicle and other parameters such as the desired application rate of the material, etc., as well as system parameters such as nozzle size, etc., is utilized to adjust the flow control valve as necessary to provide a uniform chemical application density at the desired application rate.
Such systems are advantageous in uses wherein a constant concentration of material is to be applied to a given area. However, it is somewhat difficult in such systems to vary the concentration after the liquid has been premixed in the supply tank. For example, if liquid at one concentration is being applied and it is desired to increase the concentration, an additional amount of the active chemical agent may be added to the supply tank, and some suitable means used to agitate or further mix the contents of the tank to achieve a uniform concentration. However, the operator must know approximately the volume of fluid remaining in the supply tank and perform calculations to assure arriving at the desired final concentration when adding chemical. In this regard, most applications consist of a carrier liquid such as water, which is mixed at a desired concentration, usually of no more than a few percent, with the active chemical agent of interest.
On the other hand, should the operator wish to decrease the chemical concentration, additional water may be added to the tank. Again, this requires some means of determining the remaining liquid in the tank and calculating the amount of water to be added to arrive at the desired new concentration This is, at best, a somewhat tenuous calculation and procedure.
Moreover, having added water to the tank, the operator may have considerable supply of fluid at the new concentration left over after the spraying is complete. This results in an undesirable amount of waste.
Accordingly, it has been proposed in situations where varying concentrations of a given mixture are to be applied at various times, to provide some sort of injection mixing to inject the chemical of interest at some point in the system prior to the spray boom or distribution boom. Because of the complexity and volume of the supply tubing and the overall size of such systems, there is considerable delay or lag time before a changed or new concentration being supplied at the injectors actually reaches the nozzles. The correct concentration will occur initially at the injection point and reach the nozzles only after a delay time which may be on the order of several or more seconds. For typical speeds of such sprayer vehicles, such a delay may result in a delay of from ten to one hundred or more feet of travel of the vehicle and hence a corresponding misapplication over a corresponding area.
We have recognized that the problem may be solved by the use of a carrier control based on not only the ground speed of the vehicle, but also a controlled chemical injection based on the flow rate of the carrier. That is, we have proposed a novel controller which not only controls the flow rate of liquid to the spray bar based upon the ground speed of the vehicle and the desired application rate, but also controls the rate of chemical injection into the flow in accordance with the flow rate of the carrier. Moreover, since typical chemical percentages of the carrier are on the order of a few percent, and often less than one percent, we have found that actual carrier flow need not be measured Rather, we have found that good control can be maintained by measuring the flow rate downstream of the main flow control valve to the spray bar; that is, after the point at which mixing takes place.
As a further matter, we have determined that an actual flow meter need not be provided in the system, since the carrier flow can be closely estimated by calculating the system parameters, the ground speed of the vehicle and by measuring the pressure across the spray bar. Generally speaking, it is a simpler matter to measure the pressure across the spray bar than to insert a flow rate meter in the supply line to the spray bar. In essence, our system provides a constant concentration of chemical in the system without the necessity of tank mixing. However, unlike tank mixing, our system provides for changes in desired concentration to be accommodated essentially instantly by providing the option of varying the application rate at the main flow control valve, since the system chemical concentration stays essentially constant at this point in the system. The lag time from an abrupt change in desired concentration is readily eliminated by gradually varying both the flow rate of the mixed chemical and the injection rate at the mixing point in a cooperative fashion to achieve a uniform and yet desired application rate at all times, while accommodating desired changes in concentration. That is, a changed concentration may be applied by varying application rate (flow to the nozzles or vehicle speed) as well as by changing concentration. However, we have also recognized in this regard, that the range of pressures and hence flow rates achievable with a given spray nozzle are limited. In practice, we have found that a given spray nozzle can only achieve a range of about a 2:1 ratio in the flow rate therethrough. This corresponds to about a 4:1 pressure range. This is because at much lower rates the pressure will be insufficient to maintain a true spray at the spray nozzle and the fluid will tend to "dribble". At higher flow rates, the pressure will be excessive, such that so-called "cavitation" will take place and a very nonuniform spray pattern will result.
Our novel control system, which utilizes both flow control of the flow of mixed fluid to the spray bar as well as injection control based on flow rate upstream of the main flow control valve makes it possible to accommodate these limitations of spray nozzles. That is, by taking into account the spray nozzle type and optimum range of pressures, it is possible to adjust both the flow rate to the spray bar and the injection rate into the carrier to achieve desired application rate control well within the allowable range of pressure at which fluid may be supplied and consequent flow rate which may be achieved with a given set of nozzles.
Our novel system permits great efficiency of spraying, good control over the application rate and over the desired concentration of sprayed chemical, and very little waste in spraying. In contrast to a premixed tank where all the mixture left in the tank must often be discarded after spraying, very little waste occurs in our system. With our proposed system, if a system clean-out and purging is desired, only the system plumbing need be purged and there is no relatively large tank or reservoir of premixed fluid which must be discarded or purged. That is, since the supplies of unadulterated carrier and of chemical of interest are kept separate until the injection point, only the intermediate system plumbing and lines to the spray bar need be purged or cleaned in order to accommodate changes in identity of chemical to be applied, or the like.